Dispersal of nanoparticles into or onto other media has been one of the major research thrusts during the last 5 to 10 years in the materials research community. Enhancing physical and mechanical properties of composite materials that contain nanoparticles depends on our ability to fully disperse them at small length scales. Conventional polymeric composites contain fibers or additives with length scales in the order of 5-50 μm. For example, higher performance polymeric composites contain 6-8 μm-diameter graphite fibers, whereas lower performance and cheaper composites contain 12-15 μm-diameter glass fibers. Fibers made of other materials are used less often in polymeric composites.
Nanoparticles are expected to enhance the properties of conventional composites if larger particulates and clusters are filtered out. Larger macro-particles or clusters are detrimental to the properties of the composite materials since, they: (i) tend to have various types of defects at the atomic or molecular level, (ii) may contain voids or impurities, (iii) facilitate ineffective load transfer at the interface due to their larger size, and (iv) are prone to initiating failure due to their usually irregular shape. For such cases, mixing nanomaterials into other materials, whether they are polymeric or metallic, may degrade some of the original material properties.
Methods of mixing and dispersing nanomaterials into other materials are currently being studied in the field. Various proposed methods include (i) directly mixing nanomaterials with a polymer either in the solid or liquid form by a mechanical mixer, (ii) using a sonicator or ultrasonic energy to enhance dispersion, (iii) using an electric field after mechanical mixing to enhance dispersion (applying AC or DC voltage at various levels has been tried), (iv) solid state mixing such as grinding nanomaterial and polymer together at room temperature, (v) mixing with a polymer during melting in an extruder before the extrusion or molding process (this method benefits from the high shear forces generated in an extruder to enhance dispersion), (vi) cryo-mixing or mixing at very low temperatures to make the polymer brittle enough so that polymer particles can be mixed with nanomaterials at a smaller length scale, and (vii) heating the polymer to higher temperatures to decrease the viscosity of the polymer to enhance the mixing.
The methods mentioned above are processing-based efforts to achieve uniform and nanoscale dispersion/mixing so that functional nanocomposites can be manufactured. There have been dozens, possibly hundreds, of articles published during the last five years studying various aspects of these methods.
The utilization of nanometer scale particulates such as carbon nanotubes and nanoclay with polymers is increasing due to the potential improvements in thermo-physical properties even with minor amounts. Among these nanometer scale particulates, nanoclay is widely used due to its low cost and availability. The utility of nanoclay with thermoplastics is demonstrated with various studies and commercial applications. For instance, researchers at The Toyota Research Labs observed up to 60% increase in strength of nylon 6 samples with the addition of nanoclay. Similarly, Chen et al.1, fabricated nanocomposite samples by melt-compounding nanoclay with maleic anhydride modified polypropylene at loadings up to 50%. The authors observed monotonic improvements in tensile strength and stiffness, reaching 120% and 400%, respectively. In addition to the academic research, several applications utilizing nanoclay such as in side moldings in automobiles have been commercialized.
Contrary to its utilization with neat thermoplastics, potentials of nanoclay could not previously be fully realized in thermosetting resins or thermosetting composites. The reported results in literature are often inconsistent, or indicate degradation of properties such as tensile strength by the nanoclay addition. For instance Abot et al.2 used two commercially available nanoclays, Cloisite® 30B and Nanomer® I.28E to reinforce DGEBA type epoxy resin. The mechanical properties and glass transition temperature of the fabricated samples were characterized. The tensile stiffness was observed to increase by 31% while the strength deteriorated by 28%. In addition to reduction in tensile strength, the authors observed 28% reduction in glass transition temperature. Although x-ray diffraction patterns suggested intercalation for Nanomer® I.28E and exfoliation for Cloisite® 30B, scanning electron micrographs taken at low magnification indicated existence of nanoclay aggregates as large as 10 μm. It is believed that the effect of the nanoclay aggregates suppressed the positive effects of exfoliated nanoclay platelets (nanoparticles) and resulted in reduction of strength. Lam et al.,3 on the other hand, measured the hardness of nanoclay/epoxy composites mixed in an extruder. The authors observed an increase in the hardness of the composites up to 4% nanoclay loading. However, beyond 4% loading, the hardness of the samples decreased drastically. In the same study, increasing nanoclay content is shown to yield significantly larger nanoclay cluster (aggregate) formations in the nanocomposite microstructure.
The strong tendency of nanoparticles to form clumps and clusters (“agglomerates”) is a serious technological problem that impedes the effective use of nanoparticles in many applications. Additives which can be used to disperse the nanoparticles can be useful but present their own problems, especially in regard to the purity of the final product.
Thus the inconsistent results and property degradations observed in such nanoclay/thermosetting resin composites is most likely due to insufficient (poor) dispersion and exfoliation of nanoclay platelets within the polymer matrix. Despite a number of available nanoclays with modified surfaces, complete exfoliation of nanoclay in epoxy matrices could not be achieved. Solution blending, melt mixing and in-situ polymerization have been the primary methods to introduce nanoclay into polymer matrix. However, regardless of the method, nanoclay is first mixed into the liquid resin. Several mixing methods such as ultrasonic mixing and shear mixing have been used. Among such studies a three-roll mill machine is used by Yasmin et al.4 to mix nanoclay into epoxy matrix. However expected improvements in thermo-physical properties could not be achieved. Lam et al.5 on the other hand, investigated the effect of sonication time on the hardness of nanoclay/epoxy samples. Interestingly, Lam et al. observed significant deterioration in hardness for sonication times greater than 10 minutes. Aktas et al.6 investigated the effect of several processing parameters such as mixing temperature, nanoclay type and nanoclay content on the quality of dispersion by a combination of image analysis and wavelength dispersive spectrometry. Despite certain improvements in the quality of dispersion for a set of processing parameters, complete exfoliation could not be achieved.
In view of the above, more effective methods of dispersing/mixing nanoparticles on or in substrates or base materials is therefore highly desired.